1. Field of the Invention
This invention relates to oil shale retorting, and more particularly to an improved retorting process that is energy efficient and which produces an upgraded syncrude product.
2. The Prior Art
Above-ground shale retorts can be grouped into three categories.
In the first category, the retorts are heated by a flow of internal combustion gases through the bed. In this type process, thermal efficiencies are high because energy is recovered from the retorted shale by combusting the carbonaceous residue from the retorting step. However, shale oil recoveries are rather low, heat transfer is slower than for a solid-solid system, and the gas produced is of low heating value.
In the second category, the oil shale is contacted with flowing gases that have been heated outside the retort. Thermal efficiencies are rather low for this type process because energy is not recovered from the residual carbon. However, oil recovery is quite high, and a high heating value gas is produced.
In the third category, retort heat is provided by solid-solid contact in which oil shale is contacted with hot spent shale or other solid heat carrier from a combustor or other type of solids heater. High oil recoveries result, and a high heating value gas is produced. However, the oil shale feed must be contacted with large amounts of hot spent shale to provide adequate retort heat, and a large amount of fines are carried over with the retort vapors presenting liquid cleanup problems.
Many variations of oil shale retorting processes are described in the prior art. U.S. Pat. Nos. 3,044,859; 3,440,162; 3,573,197; 3,784,462; and 4,087,347 are representative of this prior art. These patents describe variations of solid-solid heat exchange processes including use of bucket elevators to move hot char (U.S. Pat. No. 3,573, 197) and return of heavy oil from a distillation column to the retorting zone (U.S. Pat. No. 3,440,162).